The present invention relates to a current-controlled oscillator suitable for constituting an FM modulator. More particularly, the present invention relates to a current-controlled multivibrator which compensates for effects due to temperature.
FIG. 1 is a circuit diagram of a current-controlled oscillator of a conventional emitter-coupled astable multivibrator, which is equivalent to that disclosed in Japanese Patent Disclosure No. 59-22434.
Referring to FIG. 1, the collectors of a pair of transistors 1 and 2 are connected to the emitters of transistors 3 and 4, respectively. The collectors of transistors 1 and 2 are also connected to the emitter of transistor 5 respectively through resistors 8 and 9. The collector of transistor 1 is connected to the base of transistor 2 through the base-emitter path of transistor 6, and the collector of transistor 2 is connected to the base of transistor 1 through the base-emitter path of transistor 7.
The bases of transistors 3 and 4 are connected to voltage source 15 constituting a bias circuit, and the collectors thereof are connected to power source voltage Vcc. The emitter of transistor 6 is connected to the base of transistor 2, as described above. However, it is also branched to be grounded through current source 13. Similarly, the emitter of transistor 7 is branched to be grounded through current source 14. The emitters of transistors 1 and 2 are connected to each other through capacitor 12 and are also grounded respectively through control current sources 10 and 11.
The operation of the current-controlled oscillator with the above arrangement will now be described with reference to the timing charts shown in FIGS. 2A to 2D. In the following description, changes over time in collector potentials VC1 and VC2 and emitter potentials VE1 and VE2 of transistors 1 and 2 will be explained.
The operation of the current-controlled oscillator is performed by repeating alternate ON/OFF inversions of transistors 1 and 2. Assume that transistor 1 is turned on and transistor 2 is turned off at time t1. At this time, since transistor 3 is turned on, collector potential VC1 of transistor 1 is lower than base-emitter voltage VBE3 of transistor 3. Assuming that a potential difference between output terminal 16 and the base of transistor 5 is VO, the potential of output terminal 16 is VCC-VO, and collector potential VC1 of transistor 1 at time t1 is given by: EQU VC1=VCC-VO-VBE3(t=t1) (1)
Collector potential VC2 of transistor 2 is lower than power source voltage VCC by base-emitter voltage VBE5 of transistor 5 since transistor 4 is turned off, and can be expressed by: EQU VC2=VCC-VBE5(t=t1) (2)
Emitter potentials VE1 and VE2 of transistors 1 and 2 at time t1 are now considered. If the base-emitter voltage of transistor 7 is given by VBE7, and the base-emitter voltage of ON transistor 1 is given by VBE1, emitter potential VE1 is given by: EQU VE1=VCC-VBE5-VBE7-VBE1(t=t1) (3)
At this time, as indicated by a solid arrow in FIG. 1, input current Iin flows through capacitor 12.
Capacitor 12 is charged at a constant rate in a polarity indicated by the solid arrow, and emitter potential VE2 of transistor 2 is then decreased at a constant rate after time t1, as shown in FIG. 2D. When emitter potential VE2 is decreased and base-emitter voltage VBE2 of transistor 2 reaches a certain value, transistor 2 is inverted from OFF to ON. If the value of base-emitter voltage VBE2 causing this inversion is given by VBE2(ON) and the time of inversion is t2, emitter potential VE2 of transistor 2 at time t2 is expressed by: EQU VE2=VB2-VBE2(ON)(t=t2) (4)
In this case, VB2 is the base potential of transistor 2. Since VB2 is lower than collector potential VC1 of transistor 1 by base-emitter voltage VBE6 of transistor 6, VB2 is expressed by: ##EQU1## As a result, this yields: EQU VE2=VCC-V0-VBE3-VBE6-VBE2(ON)(t=t2) (6)
When transistor 2 is inverted from OFF to ON, transistor 1 is inverted from ON to OFF. Therefore, collector potential VC1 of transistor 1 is given by: EQU VC1=VCC-VBE5(t=t2) (7)
Collector potential VC2 of transistor 2 is given by: EQU VC2=VCC-VO-VBE4(t=t2) (8)
Similar to emitter potential VE1 of transistor 1 at time t1, emitter potential VE2 of transistor 2, which is turned on at time t2, is expressed by the following relation: EQU VE2=VCC-VBE5-VBE6-VBE2(t=t2) (9)
where VBE2 is the base-emitter voltage of ON transistor 2.
At time t2, emitter potential VE2 of transistor 2 increases from a value given by: EQU VCC-VO-VBE3-VBE6-VBE2(ON) (10)
to a value given by: EQU VCC-VBE5-VBE6-VBE2 (11)
by a value given by: EQU VO+VBE3-VBE5+VBE2(ON)-VBE2 (12)
If resistor 8 is selected to equalize currents flowing through the collectors of transistor 3 and 5 when transistor 3 is kept ON, base-emitter voltages VBE3 and VE5 of transistors 3 and 5 are equal to each other.
An increment of emitter potential VE2 of transistor 2 at time t2 is given by:
If EQU VBE3=VBE5 (14)
then, the increment is: EQU VO+VBE2(ON)-VBE2 (15)
Emitter potential VE1 of transistor 1 at time t2 is higher than a potential given by the following relation by the above increment: EQU VCC-VBE5-VBE7-VBE1 (16)
and is expressed by: EQU VE1=VCC-VBE5-VBE7-VBE1+VO+VBE2(ON)-VBE2(t=t2) (17)
After time t2, when transistor 1 is turned off and transistor 2 is turned on, since input current Iin flows in the direction indicated by a broken arrow in FIG. 1, emitter potential VE1 of transistor 1 then decreases at a constant rate, as shown in FIG. 2C. If the base-emitter voltage necessary for turning transistor 1 from OFF to ON is given by VBE1(ON), transistor 1 is turned on again, and transistor 1 is turned off at a time (time t3) when potential VE1 is: EQU VE1=VCC-VO-VBE4-VBE7-VBE1(ON) (18)
Emitter potential VE1 of transistor 1 which is turned on is recovered to a value given by: EQU VE1=VCC-VBE5-VBE7-VBE1(t=t3) (19)
and, an increment at time t3 is: EQU VO+VBE4-VBE5+VBE1(ON)-VBE1 (20)
If the resistance of resistor 9 is selected to equalize base-emitter voltages VBE4 and VBE5 of transistors 4 and 5, the increment of VE1 at time t3 is: EQU VO+VBE1(ON)+VBE1 (21)
Thereby, emitter potential VE2 of transistor 2 at time t3 is higher than that before time t3 given below by the above increment: EQU VCC-VBE5-VBE6-VBE2 (22)
and is: EQU VE2=VCC-VBE5-VBE6-VBE2+VO+VBE1(ON) (23)
Thereafter, ON/OFF inversion of transistors 1 and 2 is repeated, collector potential VC1 of transistor 1 or collector potential VC2 of transistor 2 has a pulse-like waveform repeated at constant interval 2T, as shown in FIGS. 2A and 2B, and the oscillation output of the current-controlled oscillator is obtained. In this case, in accordance with a change in interval T of collector potential VC1 of transistor 1, the terminal voltage of capacitor 12 changes within interval T from a value given by: EQU VCC-VO-VBE4-VBE7-VBE1(ON) (24)
to a value given by: EQU VCC-VBE5-VBE7-VBE1+VO+VBE2(ON)-VBE2 (25)
by a value given by: EQU 2VO+VBE1(ON)-VBE1+VBE2(ON)-VBE2 (26)
If the arrangement of transistors 1 and 2 is selected to be symmetrical, since EQU VBE1=VBE2, VBE1(ON)=VBE2(ON) (27)
a change in voltage applied to capacitor 12 is: EQU 2(VO+VBE1(ON)-VBE1) (28)
Therefore, if the capacitance of capacitor 12 is given by C, the following relation is established: EQU C.multidot.2(VO+VBE1(ON)-VBE1)=Iin.multidot.T (29)
Thus, oscillation frequency f0 of the current-controlled oscillator can be expressed by: EQU f0=1/2T=Iin/4CVC (30)
where VC is the terminal voltage of capacitor 12 when the ON/OFF states of transistors 1 and 2 are inverted, and is given by: EQU VC=VO+VBE2(ON)-VBE1 (31)
As can be seen from relation (30), when input current Iin is carried, oscillation frequency f0 changes. Therefore, when a video signal, for example, is used as an input signal and constant current Iin is changed by the input signal, the current-controlled oscillator shown in FIG. 1 can be used as an FM modulator for a VCR (Video Cassette Recorder).
Oscillation frequency f0 of the emitter-coupled astable multivibrator has a temperature drift, however, as will be explained below.
Temperature drift of oscillation frequency f0 is attributed to temperature drift of terminal voltage VC represented by relation (31). More specifically, at time t2 when transistor 2 is inverted from OFF to ON, the collector currents of transistors 1 and 2 are not equal to each other in practice. For this reason, a difference between temperature coefficients of base-emitter voltages VBE1 and VBE2(ON) of transistors 1 and 2 occurs, thus causing temperature drift of terminal voltage VC.
In order to understand this in more detail, collector current IC2(ON) flowing through transistor 2 will be considered.
Assuming that emitter potential VE2 is decreased by .DELTA.V and transistor 2 in the OFF state is about to be turned on before time t2, at this time, a current flowing through resistor 9 is increased by .DELTA.V.multidot.gm2 if the transconductance of transistor 2 is given by gm2. Collector current VC2 of transistor 2 is thereby decreased by .DELTA.V.multidot.gm2.multidot.R9. A change in collector potential VC2 is positively fed back to the emitter of transistor 2 itself sequentially through the base-emitter paths of transistors 7 and 1 and capacitor 12.
A condition for inverting transistor 2 from OFF to ON is that loop gain G, given by the following relation, is more than 1: EQU G=.DELTA.V.multidot.gm2.multidot.R9/.DELTA.V=gm2.multidot.R9(32)
Therefore, at an instant when G=1 (time t2), transistor 2 is inverted from OFF to ON. The collector current at this instant is IC2(ON), and transconductance gm2 is expressed by: EQU gm2=(q/KT).multidot.IC2(ON) (33)
Where K is the Boltzmann's constant, T is the absolute temperature, and q is the unit charge. From G=1 and relations (32) and (33), upon inversion of transistor 2, the collector current of transistor 2 is expressed by: EQU IC2(ON)=KT/q.multidot.R9 (34)
Since collector current IC2(ON) is obtained as described above, if a saturated current of transistors 1 and 2 is given by IS, base-emitter voltage VBE2(ON) upon inversion of transistor 2 is expressed by: ##EQU2## Since collector current IC1 of transistor 1 is substantially equal to Iin, base-emitter voltage VBE1 upon inversion of transistor 1 is expressed by: EQU VBE1=(KT/q).multidot.ln (Iin/IS) (36)
Therefore, from relations (35), (36), and (31), terminal voltage VC of capacitor 12 upon inversion is given by: EQU VC=VO-(KT/q).multidot.ln (qR9 2Iin/KT) (37)
As can be seen from the above relation, terminal voltage VC is a function of temperature T and has a negative temperature drift. Therefore, oscillation frequency f0 expressed by relation (30) has a positive temperature drift.